1. Field of the Invention
This invention relates to a semiconductor laser and its manufacturing method. Particularly, the invention relates to the semiconductor laser with an optical confinement layer.
2. Related Prior Art
With the progress in the optical communication and the optical information processing, a potential demand the semiconductor laser used in those application has grown. In particular, a semiconductor laser with a SCH structure (Separate Confinement Hetero-structure), in which the optical confinement layer and the carrier optical confinement layer are separated, is widely used because it shows a low threshold current and is easy to get a high optical output power.
Typical SCH laser has a stacking layer structure as shown in FIG. 5(a). The layers in the figure are an n-type GaInP lower cladding layer 104, an un-doped GaInAsP lower optical confinement layer 106, an un-doped GaInAs active layer 108, an un-doped GaInAsP upper optical confinement layer 110, and a p-type GaInP upper cladding layer 112. These layers are successively grown on a n-typeGaAs substrate.
FIG. 5(b) shows an typical energy band diagram of the SCH laser. In the figure, the band gap energy of the lower optical confinement layer 106 and the upper optical confinement layer 110 are larger than that of the active layer 108 by predetermined value enough to confine the carrier. Therefore, the carriers injected into the active layer 108 are effectively confined in the active layer 108.
The light generated in the active layer 108 leaks out to the lower optical confinement layer 106 and the upper optical confinement layer 110 because the thickness of the active layer is 0.1 xcexcm or less. As shown in FIG. 5(c), the refractive index of the lower cladding layer 104 and the upper cladding layer 112 are smaller than that of the optical confinement layers, the light leaking out to the optical confinement layers does not propagate in the cladding layers. Thus, the light is confined in the two optical confinement layers 106, 110 and the active layer 108 by the two cladding layers 104, 112.
The threshold current, at which the semiconductor laser starts a lasing action, is effectively reduced in the SCH laser even when the thickness of the active layer is 0.1 xcexcm or less, which is difficult in the double hetero ()H) laser without the optical confinement layer.
Moreover, since the light generated in the active layer 108 propagates not only in the carrier optical confinement layer but also in the optical confinement layers, an optical power density at the light emitting facet can be reduced and a high optical output laser can be obtained without catastrophic optical damage (COD) at the light emitting facet.
In such a SCH laser mentioned above, it is necessary to form optical confinement layers with an adequate band gap energy and refractive index. However, selecting the semiconductor material based on the band gap energy, its refractive index is automatically defined, because the refractive index and the band gap energy of a semiconductor material shows a particular relationship. Ternary, quaternary or more complicated semiconductor materials are often used for the optical confinement layer because the band gap energy of such semiconductor materials are widely adjustable by changing its composition.
It is also well known that such a ternary or quaternary semiconductor material has a miscibility gap in which the composite elements can not be mixed uniformly or the single crystal with a good quality can not be obtained. The miscibility gap widely exists in the growth temperatures from 400xc2x0 C. to 800xc2x0 C. which is typical growth condition for the semiconductor laser. It is very hard to grow a semiconductor layer with a desired band gap energy for the optical confinement layers from the composition out of the miscibility gap. Occasionally, it is impossible to get such semiconductor layers.
The object of the present invention is to provide a semiconductor laser with optical confinement layers which effectively confine both the carriers within the active layer and the light within the optical confinement layers and the active layer, and to provide a manufacturing method of the laser with such a optical confinement layers.
The semiconductor laser according to the present invention comprises a semiconductor substrate, the first and the second cladding layers on the substrate, an active layer between the first cladding layer and the second cladding layer, at least one optical confinement layer inserted between the active layer and the first cladding layer or the second cladding layer. The optical confinement layer according to the present invention includes a super lattice structure made of GaInAsP well layers and GaInP barrier layers, and each quantum well layer sandwiches a barrier layer.
In the super lattice structures, mini-bands, in which electrons or holes could move freely by tunneling effects, can be formed in the conduction band and the valence band by making the thickness of the quantum well layers and the barrier layers thin enough. The energy level difference between the mini-band of the conduction band and that of the valence band shows the same function as the band gap energy of the bulk material for the carrier confinement and the light confinement. Since the energy level difference between the mini-bands is determined by the thickness of the quantum well layers and that of the barrier layers, a super lattice structure with a desired energy level difference can be obtained by adjusting the thickness of the quantum well layers and the barrier layers. Therefore, the optical confinement layers, whose band gap energy is unrealizable in a bulk material due to the miscibility gap, can be obtained without difficulty by making the super lattice structure with the well layers and the barrier layers. Both layers are grown from the composition out of the miscibility gap under the growth temperature. GaInAsP and GaInP is the appropriate combination for the well layers and the barrier layers, respectively, because each material has the same growth conditions and the interface between them is hard to deteriorate.
The semiconductor laser according to the present invention has the energy level difference from 1.6 eV to 1.8 eV of the mini-bands in the optical confinement layer.
The method of manufacturing a semiconductor laser according to the present invention, which containing a semiconductor substrate, the first and the second cladding layer on the substrate, an active layer between the first cladding layer and the second cladding layer, and at least an optical confinement layer between the first cladding layer and the active layer or the second cladding layer and the active layer, has a step for forming an optical confinement layer with a super lattice structure made of a plurality of the GaInAsP quantum well layers and a plurality of the GaInP barrier layers.
By employing the super lattice structure for the optical confinement layer, the energy level difference between the mini-band of the conduction band and that of the valence band show the same function as the band gap energy of the bulk material for the carrier confinement and the light confinement. Since the energy level difference between the mini-bands is determined by the thickness of the quantum well layers and that of the barrier layers, a super lattice structure with a desired energy level difference can be obtained by adjusting the thickness of the quantum well layers and the barrier layers. The optical confinement layers, whose band gap energy is unrealizable in a bulk material due to the miscibility gap, can be obtained without difficulty by making the super lattice structure with the well layers and the barrier layers. Both layers are grown from the composition out of the miscibility gap under the growth temperatures. Moreover, since no particular manufacturing process is needed for the above mentioned optical confinement layers, decrease of productivity and the yield attributing to the change of the process conditions can not be occurred.
The present invention will become more fully understood from the detailed description given herein-below and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.